This invention relates to electroluminescent devices and, more particularly, to assemblies for use in electroluminescent devices and electroluminescent devices incorporating such assemblies.
Such electroluminescent devices may be constructed by depositing on a surface of a transparent substrate of, for example, glass, a transparent layer of an electrically-conductive material such as tin oxide. The unwanted portions of this layer are then removed to provide an electrode of the desired configuration having areas defining the regions of the electroluminescent device which may be required to emit light, a conductive strip adjacent an edge of the substrate and leads appropriately connecting the conductive strip to the said areas of the electrode. The electroluminescent layer is applied to the exposed surface of the electrode in the form of paint comprising an electroluminescent powder mixed with a suitable binder. After curing or drying of this paint, it is covered by an electrically-conductive layer of, for example, aluminum to provide the other electrode of the device and the device is then encapsulated for protection purposes. At this stage the device will not emit light and to cause such electroluminescent it must undergo a forming process which changes appropriately the structure of the electroluminescent layer. This is achieved by applying a unidirectional voltage to the device using a transparent layer as the positive electrode until the required structure is provided, causing the resistance of specific portions of the electroluminescent layer to increase, the current flow to fall and light to be emitted from the said regions. Thereafter the application of a suitable relatively low voltage across the electrodes will cause immediate emission of light.
The function and construction of the electroluminescent layer, the process used to form the electroluminescent layer and the operation of the electroluminescent devices have been described in detail in a number of articles and other publications. Two such articles are; "Direct-Current Electroluminescent in Zinc Sulphide : State of the Art" in Proceedings of the IEEE, Vol. 61, No. 7, July 1973 at pages 902 to 907, and an article entitled "Materials control and d.c. electroluminescence in ZnS: Mn, Cu, Cl powder phosphors" in Brit. J. Appl. Phys. (J. Phys.D), 1969, Ser. 2, Vol. 2, at pages 953 to 966.
Typical forming currents are in the region of 100mA/sq.cm. with voltages of the order of 15 to 80 volts depending upon the construction and shape of the layers of the device and, in particular, the shape of the transparent electrode. For example, higher forming voltages are necessary especially when the transparent electrode has relatively long leads connecting its conducting strip to the areas of that electrode defining the light emitting regions of the device, and in these circumstances the heat dissipated in the connecting leads can cause overheating and/or cracking of the substrate, especially when the light emitting regions are relatively large, as well as burning of the transparent electrode and the electroluminescent layer.